Misleading signature of MqttClient.ConnectAsync, MqttClientConnectResultCode expected but throws exception

[jschweda]

Describe the bug

ConnectAsync returns an MqttClientConnectResult which holds an MqttClientConnectResultCode. MqttClientConnectResultCode can indicate all kinds of errors while establishing a connection. But different than expected an exception is thrown.

Which component is your bug related to?

• Client

To Reproduce

Steps to reproduce the behavior:

1. Using Version 4.1.4.563 of MQTTnet
2. Run this server code

 var factory = new MqttFactory();
        var serverOptions = factory.CreateServerOptionsBuilder().WithDefaultEndpoint();
        _mqttServer = factory.CreateMqttServer(serverOptions.Build());
        _mqttServer.ValidatingConnectionAsync += (e) => {
            e.ReasonCode = MQTTnet.Protocol.MqttConnectReasonCode.BadUserNameOrPassword;
            return Task.CompletedTask;
        };
        _mqttServer.StartAsync().GetAwaiter().GetResult();

4. Run this client code

            _mqttClient = _mqttFactory.CreateMqttClient();
            var mqttClientOptions = _optionsBuilderFactory.CreateClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500);

            // throws unexpected exception
            MqttClientConnectResult result = await _mqttClient.ConnectAsync(mqttClientOptions, ct).ConfigureAwait(false);
            if (result.ResultCode != MqttClientConnectResultCode.Success)
            {
                _logErrorCannotConnect(_logger, result.ReasonString ?? "Unknown", result.ResultCode, null);
                return result;
            }

6. ConnectAsync is not expected to throw an exception based on signature.

Expected behavior

ConnectAsync should return a MqttClientConnectResult that has the ResultCode set to a value indicating non success.

[chkr1011]

I added a new option to the client options so that you can choose. Otherwise it would introduce breaking changes.